The present invention relates in general to a digital television receiver in which a television signal is converted into a digital signal before being processed. More particularly, the present invention is concerned with a signal processing circuit for the digital television receiver capable of performing signal processing correctly even on a nonstandard television signal which is reproduced by a video tape recorder, optical disk player or the like and which does not conform to the specification of the standard television signal.
In conventional television receivers, it is known that disturbance such as cross-color, dot-crawl and similar phenomena take place due to the frequency-multiplexing of chrominance signal on the luminance signal. Additionally, deterioration in picture quality such as line flicker, scanning line interference or the like is brought about due to the interlaced scanning. With a view to eliminating the factors involved in the deterioration in the picture quality such as mentioned above thereby ensuring a high quality in the reproduced picture, there has been proposed an apparatus in which a digital signal processing technique is utilized in combination with the use of a semiconductor memory. This apparatus usually includes a frame comb filter for separating the luminance signal and the chrominance signal from each other by making use of an inter-picture temporal correlation (frame correlation or field correlation) and a signal processing circuit for generating scanning signals sequentially by doubling the number of scanning lines through interpolation thereof with the picture or video signal belonging to different scanning lines. These circuits are disclosed in Japanese Patent application Laid-Open Nos. 115995/1983 and 79379/1983 (JP-A-No. 58-115995 and JP-A-No. 58-79379). However, the signal processing technique implemented in these circuits is only effective in the processing for generating the still picture in which a plurality of frame signals and a plurality of field signals exhibit strong cross-correlations, respectively. In case of the signal processing for motion picture, the signal processing technique undesirably gives rise to occurrence of interference signals. To deal with this problem, there is known a circuit designed to produce a signal representative of the difference in the picture (video) signal between two adjacent frames for the purpose of detecting the motion (change) of the picture. When a picture or video signal is determined for the still picture by the above-mentioned detecting circuit, then the signal processing is performed along the time axis by using the frame comb filter and the inter-field interpolation circuit. On the other hand, when the picture (video) signal is determined for the motion picture, an intra-field spatial processing is performed on the field signal. The so-called motion-adaptive processing circuit is known from Japanese Patent Application Laid-Open No. 45770/1984 (JP-A-No. 59-45770).
The technique mentioned above is certainly effective for processing a television signal whose chrominance subcarrier frequency f.sub.SC, horizontal scanning frequency f.sub.H and vertical scanning frequency f.sub.V are exactly at respective predetermined values (this television signal is hereinafter be referred to as the standard television signal or simply as the standard signal). However, difficulty is encountered in processing effectively the television signal whose chrominance subcarrier frequency f.sub.SC, horizontal scanning frequency f.sub.H and vertical scanning frequency f.sub.V are not at the predetermined values as in the case of the television signal produced by the video tape recorder (VTR) for domestic or home use, personal computer or the like (this signal is hereinafter referred to as the nonstandard television signal or simply as the nonstandard signal).
The chrominance subcarrier frequency f.sub.SC is so determined as to bear such relationship to the horizontal scanning frequency f.sub.H which is given by: ##EQU1## On the other hand, the horizontal scanning frequency f.sub.H bears a relationship to the vertical scanning frequency f.sub.V which is given by: ##EQU2## The expression (2) indicates that the scanning lines are interlaced such that there is interposed just at a mid point between two pixels (picture elements) on two adjacent scanning lines in the current field signal a pixel on the scanning line of the preceding field. On the other hand, the following expression can be derived from the abovementioned expressions (1) and (2). ##EQU3## This expression (3) shows that the chrominance subcarrier is inverted in phase at every interval equal to one frame period. As will be seen, the relations mentioned above hold true for the standard television signal, which thus can undergo the signal processing by using the frame comb filter and the inter-field signal interpolation.
However, in the case of the nonstandard signal having the frequencies f.sub.SH, f.sub.H and f.sub.V which do not satisfy the conditions given by the expressions (1) and (2), the correct pixel positioning between the two fields and the inter-frame phase inversion of the chrominance subcarrier can not take place correctly. As a consequence, scanning line interpolation by using the signal of two fields as well as separation of the luminance and chrominance signals by means of the comb filter cannot be performed accurately. Thus, when the nonstandard picture signal is decided to be a still picture signal, remarkable deterioration will be involved in the picture quality. In this manner, with the prior art circuit, difficulty is encountered in processing the nonstandard signal correctly and appropriately.
On the other hand, in order that the scanning operation in the picture tube be performed with the signal having undergone the scanning line interpolation, a signal having the horizontal scanning frequency of f.sub.H contained in the input television signal must be extracted to serve as a standard signal for generating a signal having a doubled frequency 2f.sub.H for establishing synchronization in the deflection circuit of the picture tube. Ordinarily, the signal of frequency 2f.sub.H is generated by a phase-locked loop (PLL) circuit operating in synchronism with the synchronizing signal separated from the input television signal and employed as the reference signal for the signal processing. The deflection circuit is controlled by an automatic frequency control (AFC) circuit to which the signal of the frequency 2f.sub.H is supplied. Usually, this AFC circuit also includes a PLL circuit. Accordingly, the deflection circuit is caused to synchronize with the synchronizing signal of the frequency f.sub.H at the frequency of 2f.sub.H by way of the first and second cascaded PLL circuits. The nonstandard signal generated by a VTR for home use contains appreciably jitter and skew components. Consequently, the regenerated synchronizing signal is poor in the stability. When the input signal contains the skew component (step-like change in the phase of the synchronizing signal), the first PLL circuit makes a vibratory response in an effort to follow up such change in the input signal. The second PLL circuit in turn makes a more intensive vibratory response in order to follow up the output signal of the first PLL circuit. In this way, a lot of delay is involved until the deflection circuit has attained the state to follow the change in the phase of the input signal perfectly. Further, when the synchronizing signal contains jitter components appearing irregularly, each of the PLL circuits operates with a vibratory response to the synchronizing signal containing the jitter, as a result of which the latter appears in the picture displayed on the faceplate of the picture tube in the form of flutter. As will now be understood, the prior art digital television receiver suffers a problem that the deflection circuit is poor in stability when processing the nonstandard signal produced by the home VTR for home use or the like system.